When mice lacking immune cells still sprouted grey hair, they turned to the stress hormone cortisol.

"Once they're gone, you can't regenerate pigment any more - the damage is permanent". Inside a follicle, there are stem cells that can be programmed to turn into the cells that colour hair as it regenerates; these are called melanocyte stem cells. However, when exposed to sympathetic norepinephrine, all stem cells are activated and convert to pigment-producing cells, "said Ya-Chieh Hsu, associate professor of stem cell and regenerative biology at Harvard University and a Harvard stem cell director".

The team published its report January 22 in the journal Nature.

"When we started to study this, I expected that stress was bad for the body - but the detrimental impact of stress that we discovered was beyond what I imagined", Hsu said.

Stress can reduces pigment-forming stem cells that give our hair its colour, the research discovered.

The connection between stress and the nervous system and pigment-producing cells may be an evolutionarily conserved association among different species-cephalopods, including squid and octopi, also change color when under stress. Most of us will go gray over time, regardless of whether we're living idyllic, stress-free lives or are constantly under the gun, but new research suggests that the amount of stress a person grapples with can dramatically speed up the rate at which those silver hairs appear.

The study published in Nature found that stress activates nerves that are part of the fight or flight response and permanently damage pigment-regenerating stem cells. But researchers said Wednesday they have figured out how it happens: It is driven by the body's "fight-or-flight" response to danger.

When their hearts were beating faster, their blood pressure rose, which affected their nervous system and caused acute stress.

But, as it turns out, a specific type of stress associated with the brain's response to fighting or flight is the culprit of graying. The team started their researcher with a whole-body response and progressively zoomed in on individual organ systems, cell-to-cell interaction, and eventually, molecular dynamics.

Few days into the study, the researchers found that all of the pigment-regenerating stem cells were lost. They are like bands that wrap around every hair follicle and are very close to the melanocyte stem cells. This understanding will pave the way for new studies that seek to modify or block the damaging effects of stress. Whether something similar (but in reverse) is at work in mammals and greying hair isn't clear yet, but these findings are a reminder that stress has widespread effects on the body, and may be working in ways that doctors still don't fully understand. "Understanding how our tissues change under stress is the first critical step toward eventual treatment that can halt or revert the detrimental impact of stress".